Maintaining focal length is the least of your problems. A mirror for a telescope needs to be more accurate on the order of probably a hundred to a thousand times (off the top of my head). That mirror would have horrendous astigmatism even if it was somewhat parabolic.

This idea has been around a while, with at least one guy who actually made a telescope around the idea. A 4" version worked passably at long focal lengths. The larger version, 20" I think didn't-- If memory serves, the outcome of the 'study' was that the natural shape of a surface formed this way is hyperboloidal, not paraboloidal. Fine for making a solar furnace, but not for the extremely critical focus necessary for astronomy.

This method to form a telescope mirror has already been tried ... it was shown at Stellafane back in the 80's, I believe. Unfortunately, the figure formed by the partial vacuum is no where near a parabola. The edge of the mirror is too steep and the center is too shallow. Star images will be spread across the FOV by approximately 2º. It looks cool ... but, it doesn't work!

As stated this idea has been around since the mid 1970's. There was an article in Sky and Tel by Maurice Gavin who posts here on a regular basis. Besides the fact the shape that is formed is not a very good parabola, one also has the problem that a minor change is air pressure causes the surface to change as well since you have a made a very sensitive barometer.

Good idea, at least the kernel. The specific application needs design to address each one of those problems. Just tune it up a little bit and make it really BIG.
Yeah, its not accurate to millionths of an inch but it has a 2 meter aperature! I think a camera at the focus would be the best method.
Mylar's got potential!

Good idea, at least the kernel. The specific application needs design to address each one of those problems. Just tune it up a little bit and make it really BIG.
Yeah, its not accurate to millionths of an inch but it has a 2 meter aperature! I think a camera at the focus would be the best method.
Mylar's got potential!

Potential for what? Do you understand that the rms error of a mirror is measured in nanometers deviation from a parabola over the entire surface?

that guy is crazy, did you see his other videos. before I clicked the link I thought I wonder if it's that crazy guy doing it and I clicked the link and viola.

He's so crazy. watch him cut a circle on a table saw. He's really crazy.

Actually it is a very good method for cutting circles, have done it for many years IMO gives a better circle than a router.
What does scare me about his methods is cutting from the side of the saw opposite from the fence, Now That IS an accident waiting to happen!

Hey quantumphysicist, it's EthanNino,the current top commenter on that video you linked:

Dan, are you going to continue making a telescope out of this?

I'm an engineering student, and my dream is to design an inflatable space telescope using a similar pressure differential system. When I suggested an inflatable mirror to one of my professors, he warned me that it might not be parabolic enough to reflect an image properly, but I still want to try this out. Thank you for these educational videos!

I've already begun work on a dobsonian reflector mirror using his method of stretching out Mylar sheets over a rigid drum. That's to say I'm in the very early stages of designing the mirror that MIGHT work. I should warn you in advance, do not expect too much out of this project, for the reasons commenters here have already brought up here and my early forays. I can tell just by looking at it that there's a lot of wavy imperfections on the surface, it just ain't smooth. It looks pretty good when I stare directly into it, but that might just be because it's a mirror image of me, heh. The entire surface appears to be covered in these small wavy stretch marks that are increasingly apparent when looking at the mirror from greater than a 45 degree angle. It's hard to explain, I may post images of it later.

That being said, I wouldn't give up on the notion of an inflatable telescope mirror/inflatable space telescope. Mylar is NOT the ideal material to make a telescope mirror! I mean, I used a thermal blanket I had folded up for years, so it's not by any stretch the best setup (no pun intended). I'm sure if we got some big brains on the problem of developing a reflective membrane material, we could find something more workable. There's probably already something better, we just don't know it yet. Also, I'm kind of a noob when it comes to making mylar telescope mirrors, could someone please link me to how to perform a Ronchi/Foucault test, or something equivalent to them that I can do for cheap?

Also, Dan h, Dan Rojas from the Youtube vid said in the comments section that it was a 'first' mirror. I don't really know what that means, but here's what he said when asked by another commmenter if it would make sense to make a telescope out of one of these mirrors:

Maybe, I am testing them. Our mirrors on our site are second surface so while they produce a good image they have optical aberration for telescopes. Not a problem for solar but screws up good image. The mylar is first surface on one side. We are making some first surface mirrors from acrylic.

The image quality is good from the center 50% of the mirror, the outer 50% screws it up. For solar cooking it does not matter as light is light so long as it covers the cooking vessel but, I have shrouded part of the mirror and it helps dramatically while also reducing the collection area. Since we are using cheap materials, I am thinking a 40" mirror could make a working 20" mirror 50" = 25" and so on. CPF bulb images very good.

Granted, I don't think he's constructed an entire reflector telescope since yesterday for his trashcan lid mirror, I'm pretty sure he tested the image quality using objects at close range, nevertheless it gives me hope!

Wouldn't it just be, just, awesome if we could make gigantic reflector telescope mirrors for under 12 dollars?

Henry Ford never hired experts, because they were always telling him what couldn't be done. Instead he found those who weren't aware that a task was impossible so they could do it. Think about it. Its not impossible. There's a solution for each of those problems.

The problem (with focusing) is using air pressure to maintain the shape. Since air pressure changes that won't work except for short intervals when you have to refocus each time.But using air pressure to create the shape is a good start.Then there's the shape itself, which needs to be correct.The cool thing about this type of mirror is that you don't have to send it out to get aluminized, you have a reflective layer that you can deposit easily or remove and change.

Henry Ford never hired experts, because they were always telling him what couldn't be done. Instead he found those who weren't aware that a task was impossible so they could do it. Think about it. Its not impossible. There's a solution for each of those problems.

There are plenty of failed projects out there, you can be sure that Henry had his share. I have worked on a few myself.

I am no expert but have enough common sense to recognize a doable project from a wild goose chase. That's why I am called an engineer and not a scientist.

that guy is crazy, did you see his other videos. before I clicked the link I thought I wonder if it's that crazy guy doing it and I clicked the link and viola.

He's so crazy. watch him cut a circle on a table saw. He's really crazy.

Actually it is a very good method for cutting circles, have done it for many years IMO gives a better circle than a router.
What does scare me about his methods is cutting from the side of the saw opposite from the fence, Now That IS an accident waiting to happen!

This guy is the perfect poster child for the old saying: "A little knowledge is a dangerous thing!"

Yes, his first 4-wheel vehicle was 12 years and several attempts before the Model T in '08. (From wiki research this morning.) This may have been why he didn't put much faith in experts. Without regard to any of that, a forum is where ideas fly and perhaps find appreciative members as well as critics who cite good reasons. This is a healthy environment!

PS. Can someone please tell me or link me to instructions on how to do a Ronchi/Foucault Test or something equivalent that I can do for cheap?

Also, KerryR, do you know the guys name? Or do you know if any of his work is online somewhere? It sounds interesting that he at least got a 4" mirror working.

Skyguy, do you know the guys name who tried this? Or if his work is online somewhere? I'd be interested in that Sky and Tel article too, what's Maurice's screen name here?

Dan Rojas from the YouTube video said in his reply to me that his mirror's center 50% works, while the outer 50% does not, so if you had a 10 inch mirror, only the inner 5 inches would work. I don't think this means he made a reflector telescope already out of it, but might not using the outer 50% area, would this help reduce the astigmatism?

Gavin article is in May 1979 SKy and Tel, "Aluminized Mylar as a Flux Collector"
Here is a link to more recent article about the idea. At conclusion of the article, the best figure that they could produce had 10 waves of error in it. http://www.gravic.co...Holenstein-P...

Good idea, at least the kernel. The specific application needs design to address each one of those problems. Just tune it up a little bit and make it really BIG.
Yeah, its not accurate to millionths of an inch but it has a 2 meter aperature! I think a camera at the focus would be the best method.
Mylar's got potential!

Before you can begin to evaluate the potential of mylar, you are going to have to eliminate the other sources of error. First and foremost is that the supporting rim of the mirror needs to be at optical tolerances if you expect the mylar surface to be at optical tolerances. A trash can lid is not going to do it. You need to machine and polish a rim to support the mylar accurately. Then determine how to manage the thermal expansion of this rim. Next you need to control atmospheric pressure, etc. etc.. Lots to work on before you can get to testing the mylar but in the meantime, you can cook some hotdogs with your setup.

Ed- it might be an irregular surface, sort of a sphere, sort of a catenary- because of the mylar's stretching.If there is a foundation for the mylar all you need to do is attach it with spray adhesive and and hold the vacuum long enough to solidify the coating to the foundation. The foundation should be an accurate parabolic curve but it doesn't have to be glass, just solid and something that can be configured to that curve. You might be able to use a lathe to make it or just grind with a tool like you do a glass mirror, only it wouldn't have to be perfect, as in polished, because you have the film for that. (And also, unlike glass you could just fill in an area that was too low instead of taking the whole curve down to match the low surface.) Still looking for a good material for the foundation. There are some rigid foams that might work...